Chapter 8
Alkenes:
Reactions and
Synthesis
Suggested Problems 1-21,26-9,32-5,37-39
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Diverse Reactions of Alkenes
• Alkenes undergo electrophilic addition reactions
to give many useful products
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Preparation of Alkenes: A Preview
of Elimination Reactions
• Alkenes are commonly made by an elimination
reaction
1.Dehydrohalogenation - Loss of HX from an
alkyl halide
• Occurs by reaction of an alkyl halide with strong
base
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Preparation of Alkenes: A Preview
of Elimination Reactions
2. Dehydration - Loss of water from an alcohol
• Carried out by treating an alcohol with a strong
acid
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Worked Example
• How many alkene products, including E,Z
isomers, might be obtained by dehydration of 3methyl-3-hexanol with aqueous sulfuric acid?
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Worked Example
• Solution:
– It is possible to obtain five alkene products by
the dehydration of 3-methyl-3-hexanol CHE2201, Chapter 8
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Electrophilic Addition Reactions
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Halogenation of Alkenes: Addition
of X2
• Halogenation - Bromine and chlorine add to
alkenes to give 1,2-dihalides
• Example
– 1,2-dichloroethane is formed by addition of Cl2 to
ethylene
• Fluorine is too reactive and iodine does not react
with majority of alkenes
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Halogenation of Alkenes: Addition
of X2
• Halogenation reaction of cycloalkane forms the
trans stereoisomer of the dihalide addition
product
• Reaction occurs with anti stereochemistry
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Mechanism of Bromine Addition
• As suggested by George Kimball and Irving
Roberts, for the observed stereochemistry the
reaction intermediate is not a carbocation
• Bromonium ion, R2Br+, is formed by
electrophilic addition of Br+ to the alkene
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Mechanism of Bromine Addition
• Bromonium ion is formed in a single step
– Interaction of the alkene with Br2 and simultaneous
loss of Br-
• Reaction with Br- ion occurs only from the
opposite, unshielded side to give trans product
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Mechanism of Bromine Addition
• Bromonium ions were postulated more than 75
years ago to explain stereochemistry of halogen
addition to alkenes
• George Olah showed that bromonium ions are
stable in liquid SO2
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Addition of a Halogen is an Anti
Addition
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Anti Addition to a Cis Isomer
Forms Only the Trans Stereoisomers
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No Carbocation Rearrangements
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Worked Example
• Addition of HCl to 1,2-dimethylcyclohexene
yields a mixture of two products
– Show the stereochemistry of each, and explain why a
mixture is formed
• Solution:
• Addition of hydrogen halides involves formation of an
open carbocation
• The carbocation, which is sp2-hybridized and planar,
can be attacked by chloride from either top or bottom
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Worked Example
– This yields products in which the two methyl
groups can be either cis or trans to each other
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Halohydrins from Alkenes: Addition
of HOX
• Reaction of alkenes with hypohalous acids HO–Cl
or HO–Br yields 1,2-halo alcohol, called a
halohydrin
• Addition takes place by reaction of the alkene with
either Br2 or Cl2 in the presence of water
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Formation of Halohydrins
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Mechanism for Halohydrin Formation
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How to Account for Regioselectivity
The electrophile adds to the sp2 carbon
bonded to the most hydrogens.
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Halohydrins from Alkenes: Addition
of HOX
• Bromohydrin formation is carried out in a solvent
such as aqueous dimethyl sulfoxide, CH3SOCH3
(DMSO), using the reagent N-bromosuccinimide
(NBS)
– Produces bromine in organic solvents and is a safer
source
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NBS Mechanism
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More Reactions
• Different nucleophiles
• Each involves a chloronium or bromonium ion
intermediate
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Worked Example
• What product would you expect from the
reaction of cyclopentene with NBS and water?
– Show the stereochemistry
• Solution:
– –Br and –OH are trans in the product
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Hydration of Alkenes: Addition of
H2O
• Hydration of an alkene is the addition of H2O to
give an alcohol
• Reaction takes place on treatment of the alkene
with water and a strong acid catalyst
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Mechanism of Hydration
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Hydration of Alkenes: Addition of
H2O
• Acid-catalyzed hydration of isolated double
bonds is uncommon in biological pathways
• Fumarate is hydrated to give malate as one step
in the citric acid cycle of food metabolism
• In the laboratory, alkenes are often hydrated by
the oxymercuration–demercuration procedure
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Hydration of Alkenes: Addition of
H2O by Oxymercuration
• Reaction is initiated by electrophilic addition of
Hg2+ ion to the alkene
– Gives an intermediate mercurinium ion
• Regiochemistry of the reaction corresponds to
Markovnikov addition of H2O
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Acid-Catalyzed Addition of an Alcohol
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Carbocation Rearrangement
(a 1,2-hydride shift)
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Carbocation Rearrangement
(a 1,2-methyl shift)
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The Carbocation Does Not Rearrange
• No Improvement in Carbocation Stability is
afforded here
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Worked Examples
• What products would you expect from
oxymercuration–demercuration of the following
alkenes?
a)
b)
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Worked Examples
• Solution:
– Oxymercuration is equivalent to Markovnikov
addition of H2O to an alkene
a)
b)
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Hydration of Alkenes: Addition of
H2O by Hydroboration
• Hydroboration: Process involving addition of a
B–H bond of borane, BH3, to an alkene to yield
an organoborane intermediate, RBH2
• Boron has six atoms in its valance shell making
borane a very reactive Lewis acid
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BH3 Contains Three Hydrides – Dialkyl and
Trialkylboranes Typically Form
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Hydration of Alkenes: Addition of
H2O by Hydroboration
• Alkene reacts with BH3 in THF solution, rapid
addition to the double bond occurs three times
and a trialkylborane is formed
• Net effect of the two-step hydroboration–
oxidation sequence is hydration of the alkene
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double bond
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R2BH Allows Only Monoalkylation
Because of its bulky R groups, it has a
stronger preference for the less substituted sp2
carbon.
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Hydration of Alkenes: Addition of
H2O by Hydroboration
• During hydroboration–oxidation of
1-methylcyclopentene, boron and hydrogen add
to the alkene from the same face of the double
bond with syn stereochemistry
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Hydroboration
• Differs from other alkene addition reactions
– Occurs in a single step without a carbocation
intermediate
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Addition of BH3 and Addition of HBr
Follow the Same Rule
• Regiochemistry that results when an
unsymmetrical alkene is hydroborated makes
the hydroboration reaction very useful
• Anti-Markovnikov
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Mechanism for the Oxidation Reaction
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Only Syn Addition
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The H and OH Add
to the Same Side of the Ring
• Hydroboration-oxidation is stereoselective – only
two of the four possible stereoisomers are formed.
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No Carbocation Rearrangements
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Worked Example
• What alkene might be used to prepare the
following alcohol by hydroboration–oxidation?
• Solution:
– The products result from
hydroboration/oxidation of a double bond
– The –OH group is bonded to the less
substituted carbon of the double bond in the
starting material
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Retrosynthesis
How would you make this molecule?
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Reduction of Alkenes:
Hydrogenation
• Hydrogenation: Addition of hydrogen to a
double or triple bond to yield a saturated product
• Reduction: Reaction that results in gain of
electron density for carbon caused either by:
– Bond formation between carbon and a less
electronegative atom (usually H)
– Bond-breaking between carbon and a more
electronegative atom (usually O,N,X)
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Reduction of Alkenes:
Hydrogenation
• Usually occurs with syn stereochemistry
• H2 is adsorbed onto a catalyst surface
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Mechanism for Hydrogen Addition
catalytic hydrogenation
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Reduction of Alkenes:
Hydrogenation
• Catalytic hydrogenation is extremely sensitive to
the steric environment around the double bond
• In α-pinene reduction occurs exclusively from
the bottom face
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Selectivity in Hydrogenation
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Reduction of Alkenes:
Hydrogenation
• Catalytic hydrogenation is important in the food
industry
• Incomplete hydrogenation results in partial cis–
trans isomerization of a remaining double bond
• In biological hydrations, biological reductions
occur in two steps:
– Reducing agent, NADPH, adds a hydride ion to the
double bond to give an anion
– Anion is protonated by acid HA, leading to overall
addition of H2
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Trans Fatty Acids Raise LDL
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Reduction of the Carbon–Carbon
Double Bond in Trans-crotonyl ACP
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Worked Example
• What products are obtained from catalytic
hydrogenation of the following alkenes?
a)
b)
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Worked Example
• Solution:
a)
b)
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Oxidation of Alkenes: Epoxidation
and Hydroxylation
• Oxidation: Reaction that results in a loss of
electron density for carbon by:
– Bond formation between carbon and a more
electronegative atom (usually O,N,X)
– Bond-breaking between carbon and a less
electronegative atom (usually H)
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Oxidation of Alkenes: Epoxidation
• Alkenes oxidize to give epoxides on treatment
with a peroxyacid, RCO3H
•
Epoxide: Cyclic ether with an oxygen atom in a
three-membered ring
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Only Syn Addition is Observed
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Mechanism for Epoxidation
The mechanism is similar to that for the addition of Br2
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Syn Addition to a Cis Isomer
Forms Only the Cis Stereoisomers
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Syn Addition to a Trans Isomer
Forms Only the Trans Stereoisomers
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Oxidation of Alkenes: Epoxidation
• Treating a base with halohydrin leads to
elimination of HX and production of an epoxide
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Oxidation of Alkenes: Hydroxylation
• Epoxides undergo an acid-catalyzed ringopening reaction with water
– Gives corresponding 1,2-dialcohol, or diol, also called
a glycol
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Oxidation of Alkenes: Hydroxylation
• The net result of the two-step alkene
epoxidation/hydrolysis is hydroxylation
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Oxidation of Alkenes: Hydroxylation
• Hydroxylation can be carried out directly by
treating an alkene with osmium tetroxide
– Reaction occurs with syn stereochemistry
– Does not involve a carbocation intermediate
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Oxidation of Alkenes: Hydroxylation
• The use of NMO as a cooxidant permits a
catalytic cycle
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Worked Example
• What product is expected from reaction of cis-2butene with meta-chloroperoxybenzoic acid (mCPBA)?
– Show the stereochemistry
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Worked Example
• Solution:
– Epoxidation using m-chloroperoxybenzoic acid (mCPBA) is a syn addition
– Original double bond stereochemistry is retained
– The methyl groups are cis
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Oxidation of Alkenes: Cleavage to
Carbonyl Compounds
• Ozone (O3) adds to C═C bond, at low
temperature, to form molozonide
• Molozonide rearranges to form ozonide
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Oxidation of Alkenes: Cleavage to
Carbonyl Compounds
• Ozonide is treated with a reducing agent to
produce carbonyl compounds (Zn/AcOH or
dimethyl sulfide)
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Ozonolysis
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What Alkene Gave these Ozonolysis
Products?
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Oxidation of Alkenes: Cleavage to
Carbonyl Compounds
• Oxidizing reagents other than ozone cause
double-bond cleavage
• Potassium permanganate (KMnO4) can produce
carboxylic acids and carbon dioxide if hydrogens
are present on C═C
• With no hydrogens, ketones are produced
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Oxidation of Alkenes: Cleavage to
Carbonyl Compounds
• Alkenes can be cleaved by hydroxylation to form
a 1,2-diol followed by reaction of the diol with
periodic acid, HIO4, to afford carbonyl compounds
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Worked Example
• What products would be expected from reaction
of 1-methylcyclohexene with aqueous acidic
KMnO4?
• Solution:
– Aqueous KMnO4 produces:
• A carboxylic acid from a C═C
• A ketone from a double bond carbon that is
disubstituted
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Addition of Carbenes to Alkenes:
Cyclopropane Synthesis
• Carbene, R2C: A neutral molecule containing a
divalent carbon with only six electrons in its
valence shell
– Electrophilic addition of a carbene to an alkene yields
a cyclopropane
• Adds symmetrically across the double bond
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Dichlorocarbene Generation Mechanism
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The Structure of Dichlorocarbene
• A carbene is planar being sp2 hybridized with a
vacant p-orbital.
• Note its similarity in structure to that of a
carbocation
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Addition of Carbenes to Alkenes:
Cyclopropane Synthesis
• Addition of dichlorocarbene with cis-2-pentene is
stereospecific
– Stereospecific: Only a single stereoisomer is formed
as product
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Simmons-Smith Reaction
• Method for preparing nonhalogenated
cyclopropanes
• Does not involve a free carbene
• Utilizes a carbenoid
• Reaction of diiodomethane with zinc-copper
alloy produces (iodomethyl)zinc iodide
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Simmons-Smith Reaction
• (Iodomethyl)zinc iodide yields the corresponding
cyclopropane in the presence of an alkene
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Worked Example
• What product is expected from the following
reaction
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Worked Example
• Solution:
– Reaction of a double bond with CH2I2 yields a
product with a cyclopropane ring that has a –
CH2– group
– Two different isomers can be formed,
depending on stereochemistry of the double
bond
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Radical Additions to Alkenes:
Chain-Growth Polymers
• Polymer: Large molecule consisting of repeating
units of simpler molecules, called monomers
– Formed by polymerization
• Alkenes react with radical catalysts to undergo
radical polymerization
• Simple alkene polymers are called chaingrowth
polymers
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Radical Additions to Alkenes:
Chain-Growth Polymers
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Radical Additions to Alkenes:
Chain-Growth Polymers
• Initiation
– A few radicals are generated on heating a small
amount of benzoyl peroxide catalyst
– Benzoyloxy radical loses CO2 and gives a phenyl
radical
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Radical Additions to Alkenes:
Chain-Growth Polymers
• Propagation
– Radical from initiation adds to alkene to generate
alkene derived radical
– Process repeats to form the polymer chain
• Termination
– Chain propagation ends when two radical chains
combine
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Radical Additions to Alkenes:
Chain-Growth Polymers
• Other alkenes give other common polymers
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Some Alkene Polymers and Their
Uses
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Worked Example
• Show the monomer units required to prepare the
following polymer:
• Solution:
– The smallest repeating unit in each polymer is
identified and double bond is added
– Monomer
• H2C═CHOCH3
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Biological Additions of Radicals to
Alkenes
• More controlled and more common than
laboratory or industrial radical reactions
• Radical addition reactions have severe
limitations in a laboratory environment
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Pathway of Biosynthesis of
Prostaglandins from Arachidonic Acid
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Reaction Stereochemistry: Addition
of H2O to an Achiral Alkene
• The laboratory hydration of 1-butene yields an
intermediate secondary carbocation by
protonation
• It reacts with H2O from either the top or the
bottom face to afford the two enantiomers
• Formation of a new chirality center by achiral
reactants leads to a racemic mixture of
enantiomeric products
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Reaction of H2O with the Carbocation
Resulting from Protonation of 1-Butene
• The two transition states are mirror images
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Reaction Stereochemistry: Addition
of H2O to an Achiral Alkene
• Optically active product can only result by starting
with an optically active reactant or a chiral
environment
• Cis-aconitate is achiral
– Only the enantiomer of the product is formed in a
biological reaction
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Reaction Stereochemistry: Addition
of H2O to a Chiral Alkene
• The stereochemistry in acid-catalyzed addition
of H2O is established by reaction of H2O with a
carbocation intermediate
– Does not contain a plane of symmetry
– Chiral because of existing chirality center
• Formation of a new chirality center by a chiral
reactant leads to unequal amounts of
diastereomeric products
• Products are also optically active, if the chiral
reactant is optically active because only one
enantiomer is used
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Stereochemistry of the Acid-Catalyzed
Addition of H2O to the Chiral Alkene
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Worked Example 1
Which reaction would one predict to be faster,
addition of HBr to cyclohexene or to 1methylcyclohexene?
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Worked Example 1
First, draw out both reactants with HBr. What we
should realize at this point is that the formation of
the intermediate that is more stabilized via
carbocation formation is the one that will form
product faster. At this point, we should see that the
intermediate formed via the 3˚ intermediate from 1methylcyclohexene (as opposed to the 2˚
carbocation intermediate in the case of
cyclohexene) will proceed faster.
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Worked Example 2
• What products are formed from hydration of 4methylcyclopentene?
– Consider all stereoisomers formed
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Worked Example 2
• Solution:
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